"""Lock-safe inspection of SQLite database files.

Why this module exists
----------------------
POSIX advisory locks are cancelled **process-wide** by ``close()`` on *any*
file descriptor for that file::

    the close() system call will cancel all POSIX advisory locks on the
    same file for all threads and all file descriptors in the process
    -- https://sqlite.org/howtocorrupt.html#_posix_advisory_locks_canceled_by_a_separate_thread_doing_close_

So a bare ``open(db_path, "rb") ... close()`` on a **live** database silently
drops every lock SQLite holds on it from this process -- including the
EXCLUSIVE lock a ``VACUUM`` is holding while it rewrites the whole file, and
the RESERVED lock an in-flight ``BEGIN IMMEDIATE`` is holding. Other processes
are then free to write into a file that a writer still believes it owns, which
is the documented route to "database disk image is malformed".

Hermes is exactly the topology this hits: gateway, dispatcher, dashboard,
TUI, CLI, cron and kanban workers all open the same ``state.db`` /
``kanban.db``, and several code paths used to byte-probe those files while
connections were live.

The rules
---------
1. **Never** ``open()`` a database file that may have live connections in this
   process. Ask SQLite instead -- :func:`page_count_bytes` reads the same
   header field via ``PRAGMA``, over the existing connection, taking no new
   descriptor.
2. Byte-level probes are only safe **before any connection exists** for that
   path (first-open validation). Route those through
   :func:`read_header_bytes_preopen`, which refuses once a connection has been
   registered for the path.

Concurrency contract
--------------------
The registry is not advisory bookkeeping -- it is the guard, so the
check and the byte read must be **atomic with respect to connection
lifecycle**. ``_live_lock`` is therefore held across three critical sections,
each of which spans the syscall *and* the registry mutation:

* open + register (:func:`connect_tracked`)
* unregister + close (:meth:`TrackedConnection.close`)
* check + ``open``/``read``/``close`` (:func:`read_header_bytes_preopen`)

Without that, a thread could pass the "no live connection" check, a second
thread could open a connection and take a write lock, and the first thread's
``close()`` would then cancel it -- reintroducing the exact bug this module
exists to prevent. The lock is never held while a caller *uses* a connection,
only across these transitions, so it does not serialise database work.

Path identity
-------------
Connections are keyed by the **canonical database path**, resolved from
``PRAGMA database_list`` on the opened connection. The caller's spelling is
not trustworthy: ``SessionDB``'s read-only path opens
``file:/…/state.db?mode=ro`` with ``uri=True``, and treating that string as a
filesystem path yields a key like ``<cwd>/file:/…/state.db?mode=ro`` which no
later probe of the real ``Path`` can ever match.
"""

from __future__ import annotations

import contextlib
import logging
import os
import sqlite3
import threading
from pathlib import Path
from typing import Optional

logger = logging.getLogger(__name__)

SQLITE_HEADER_MAGIC = b"SQLite format 3\x00"

# Offset of the 4-byte big-endian page-count field in the SQLite header.
_HEADER_PAGE_COUNT_OFFSET = 28

# Guards BOTH the registry and the lifecycle syscalls it describes. Reentrant
# because connect_tracked -> _canonical_db_path -> ... stays on one thread.
_live_lock = threading.RLock()
# canonical path -> number of live connections opened by this process
_live_connections: dict[str, int] = {}


class UntrackableConnectionError(RuntimeError):
    """A connection to a probe-able database could not be tracked.

    Raised rather than silently returning an untracked connection: on these
    paths tracking is part of the correctness contract, not an optimisation.
    """


def _key(path: Path | str) -> str:
    """Canonicalise a *filesystem* path for use as a registry key."""
    try:
        return str(Path(path).resolve())
    except OSError:
        return str(path)


def _canonical_db_path(conn: sqlite3.Connection) -> Optional[str]:
    """The on-disk path of ``main``, as SQLite itself reports it.

    Immune to the caller's spelling (``file:`` URIs, relative paths, symlinks).
    Returns ``None`` for in-memory or unnamed databases, which cannot be
    byte-probed and therefore need no tracking.
    """
    try:
        row = conn.execute("PRAGMA database_list").fetchone()
    except sqlite3.Error:
        return None
    if not row or len(row) < 3:
        return None
    path_str = row[2]
    if not path_str:
        return None
    return _key(path_str)


def track_connection(path: Path | str) -> None:
    """Record that this process now holds a connection to *path*.

    Prefer :func:`connect_tracked`; this exists for callers that manage their
    own connection objects, and for tests.
    """
    key = _key(path)
    with _live_lock:
        _live_connections[key] = _live_connections.get(key, 0) + 1


def untrack_connection(path: Path | str) -> None:
    """Record that one connection to *path* has been closed."""
    key = _key(path)
    with _live_lock:
        remaining = _live_connections.get(key, 0) - 1
        if remaining > 0:
            _live_connections[key] = remaining
        else:
            _live_connections.pop(key, None)


def has_live_connection(path: Path | str) -> bool:
    """Whether this process currently holds any connection to *path*."""
    with _live_lock:
        return _key(path) in _live_connections


class _TrackingMixin:
    """Untrack-on-close behaviour, mixable into any Connection subclass."""

    _hermes_tracked_path: str | None = None

    def close(self) -> None:  # type: ignore[misc]
        with _live_lock:
            path = getattr(self, "_hermes_tracked_path", None)
            if path is not None:
                self._hermes_tracked_path = None
                untrack_connection(path)
            super().close()  # type: ignore[misc]


class TrackedConnection(_TrackingMixin, sqlite3.Connection):
    """A ``sqlite3.Connection`` that untracks its path exactly once on close.

    Counting opens is easy; counting closes reliably is not, because callers
    close connections in many places (and some hand them to
    ``contextlib.closing``). Putting the decrement on ``close()`` — the one
    method every close path must go through — keeps the registry from
    drifting upward and permanently disabling byte-probes.

    The unregister and the real ``close()`` happen together under
    ``_live_lock`` so a concurrent probe can never observe "no live
    connection" while this descriptor is still open.

    Note ``with conn:`` does NOT close a sqlite3 connection (it only commits or
    rolls back), so this hook is not fired spuriously by transaction scopes.
    """


_tracked_factory_cache: dict[type, type] = {}


def _tracking_factory(factory: type) -> type:
    """Return *factory* augmented with untrack-on-close.

    Callers legitimately supply their own ``Connection`` subclasses (the test
    suite uses them to simulate FTS5-less or pragma-failing runtimes). Rather
    than refusing those — or silently leaving them untracked, which would
    quietly unguard the database — we mix the tracking ``close()`` into the
    caller's class so tracking is preserved either way.
    """
    if factory is sqlite3.Connection:
        return TrackedConnection
    if issubclass(factory, _TrackingMixin):
        return factory
    cached = _tracked_factory_cache.get(factory)
    if cached is None:
        cached = type(f"Tracked{factory.__name__}", (_TrackingMixin, factory), {})
        _tracked_factory_cache[factory] = cached
    return cached


def connect_tracked(
    path: Path | str,
    *,
    tracking_path: Path | str | None = None,
    connect_fn=None,
    **kwargs,
) -> sqlite3.Connection:
    """``sqlite3.connect`` that registers the connection for the lifetime of the fd.

    Use for any connection to a database whose file might otherwise be
    byte-probed (``state.db``, ``kanban.db``). The registration is released
    automatically on ``close()``.

    The open and the registration happen together under ``_live_lock``, so a
    concurrent :func:`read_header_bytes_preopen` cannot slip between them and
    cancel this connection's locks.

    The registry key is the canonical path reported by ``PRAGMA
    database_list`` -- not *path*, which may be a ``file:`` URI. Pass
    ``tracking_path`` to override when the caller already knows the real path.

    ``connect_fn`` lets a caller supply its own opener (defaults to
    :func:`sqlite3.connect`), so a module that owns the connection — and any
    test that patches that module's ``sqlite3.connect`` — keeps control of how
    the connection is created while this helper owns tracking.

    A caller-supplied ``factory`` is honoured but is transparently augmented
    with untrack-on-close, so tracking is never silently skipped. If a
    file-backed connection still cannot be tracked,
    :class:`UntrackableConnectionError` is raised rather than handing back a
    connection whose database has quietly lost byte-probe protection.
    """
    opener = connect_fn if connect_fn is not None else sqlite3.connect
    kwargs["factory"] = _tracking_factory(kwargs.get("factory", sqlite3.Connection))

    with _live_lock:
        conn = opener(str(path), **kwargs)
        try:
            resolved = (
                _key(tracking_path)
                if tracking_path is not None
                else _canonical_db_path(conn)
            )
            if resolved is None:
                # In-memory / unnamed: nothing on disk to byte-probe.
                return conn
            if not isinstance(conn, _TrackingMixin):
                # The opener substituted its own factory and discarded ours
                # (test doubles simulating FTS5-less runtimes do this). Retag
                # the instance's class with the tracking mixin so close() still
                # releases the registry entry, rather than handing back a
                # connection whose database has silently lost probe safety.
                conn = _retrofit_tracking(conn, resolved)
            conn._hermes_tracked_path = resolved
            _live_connections[resolved] = _live_connections.get(resolved, 0) + 1
            return conn
        except Exception:
            try:
                # Close via sqlite3 directly: the tracking entry was either
                # never made or is being unwound here.
                sqlite3.Connection.close(conn)
            except Exception:
                pass
            raise


def _retrofit_tracking(conn: sqlite3.Connection, resolved: str) -> sqlite3.Connection:
    """Give an already-open connection untrack-on-close semantics.

    ``sqlite3.Connection`` subclasses are ordinary Python classes, so the
    instance's ``__class__`` can be swapped for one that mixes in the tracking
    ``close()``. Used when an opener ignored the factory we asked for.
    """
    cls = type(conn)
    if issubclass(cls, _TrackingMixin):
        return conn
    try:
        conn.__class__ = _tracking_factory(cls)  # type: ignore[assignment]
        return conn
    except TypeError as exc:
        raise UntrackableConnectionError(
            f"connection to {resolved} uses factory {cls.__name__}, which "
            "cannot release its tracking entry on close; byte-probe safety "
            "for this database would be silently lost"
        ) from exc


def page_count_bytes(conn: sqlite3.Connection) -> Optional[int]:
    """Logical database size in bytes, read through *conn*.

    ``page_count * page_size`` is the same quantity the 4-byte header field at
    offset 28 carries, but reading it via ``PRAGMA`` opens no new file
    descriptor and therefore cannot cancel this process's POSIX locks.

    Returns ``None`` when the pragmas cannot be read.
    """
    try:
        page_count = conn.execute("PRAGMA page_count").fetchone()[0]
        page_size = conn.execute("PRAGMA page_size").fetchone()[0]
    except (sqlite3.Error, TypeError, IndexError) as exc:
        logger.debug("page_count/page_size unavailable: %s", exc)
        return None
    try:
        return int(page_count) * int(page_size)
    except (TypeError, ValueError):
        return None


def file_length_matches_header(conn: sqlite3.Connection) -> Optional[bool]:
    """Whether the file on disk is at least as long as the header claims.

    Detects the "torn extend" shape (file shorter than its own page count)
    without ever opening the database file: the header side comes from
    ``PRAGMA page_count`` over *conn*, and the on-disk side from ``stat()``,
    which takes no descriptor and cannot break locks.

    Returns ``None`` when the check is not applicable (in-memory database,
    unreadable pragmas, or a stat failure).

    Note: in WAL mode a freshly committed page may still live in the ``-wal``
    file, so the main file legitimately lags. Callers must treat this as
    advisory unless the database is in a rollback journal mode.
    """
    path_str = _canonical_db_path(conn)
    if path_str is None:
        return None

    logical = page_count_bytes(conn)
    if not logical:
        return None
    try:
        actual = os.path.getsize(path_str)
    except OSError:
        return None
    return actual >= logical


def read_header_bytes_preopen(
    path: Path | str,
    *,
    length: int = 100,
    force: bool = False,
) -> Optional[bytes]:
    """Read the first *length* bytes of *path* -- only when no connection is live.

    This is the ONLY sanctioned byte-level read of a database file, and it is
    restricted to first-open validation (is this file a real SQLite database,
    is it zeroed, has it been overwritten by something else). Once any
    connection to *path* exists in this process, the read is refused and
    ``None`` is returned, because the ``close()`` would cancel that
    connection's POSIX locks.

    The registry check and the ``open``/``read``/``close`` are performed
    together under ``_live_lock``, so a connection cannot be opened in the
    window between deciding "nothing is live" and closing this descriptor.

    Set ``force=True`` only for genuinely offline files (quarantined copies,
    snapshot artifacts, archives) that no live connection can reference.
    """
    with _live_lock:
        if not force and _key(path) in _live_connections:
            logger.debug(
                "refusing byte-level read of %s: a live connection exists in "
                "this process and close() would cancel its POSIX locks",
                path,
            )
            return None
        try:
            with open(path, "rb") as handle:
                return handle.read(length)
        except OSError:
            return None


class LiveConnectionError(RuntimeError):
    """A raw file operation was attempted on a database with live connections."""


@contextlib.contextmanager
def offline_file_access(path: Path | str, *, what: str = "read"):
    """Hold the connection-lifecycle lock across a raw read of a database file.

    Checking :func:`has_live_connection` and *then* doing the raw I/O is a
    check/use race: a connection can be opened in the window between the two,
    and the raw ``close()`` will cancel its POSIX advisory locks — the exact
    failure class the registry exists to prevent. Any multi-step raw access
    (copying a database plus its ``-wal``/``-shm``/``-journal`` sidecars,
    hashing a file, moving a bundle aside) must therefore run *inside* this
    context manager rather than after a bare check.

    While held, :func:`connect_tracked` blocks, so no new connection can
    appear mid-copy. Raises :class:`LiveConnectionError` if a connection is
    already live when the guard is entered.

    The lock is only held for the duration of the raw I/O; it never spans
    caller work on an open connection, so it does not serialise database use.
    """
    with _live_lock:
        if _key(path) in _live_connections:
            raise LiveConnectionError(
                f"Refusing to {what} {path}: a connection to it is still open "
                "in this process, and raw file access would cancel that "
                "connection's POSIX advisory locks. Close all database "
                "handles (stop the gateway/dashboard) and retry."
            )
        yield
